The field of this invention is magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to interactive control of the scan plane prescription during an MRI guided procedure.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0) in the z direction of a Cartesian coordinate system, the individual magnetic moments of the nuclear spins in the tissue attempt to align with this polarizing field, but precess about the field in random order at their characteristic Larmor frequency. If the substance, or tissue, is also subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment M.sub.z may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated, and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x, G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles, or "views", in which these gradients vary according to the particular localization method being used. The resulting set of received nuclear magnetic resonance (NMR) signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Intra-operative MR imaging is employed during a medical procedure to assist the physician in guiding an instrument. For example, during a needle biopsy the MRI system is operated in a real-time mode in which image frames are produced at a high rate so that the physician can monitor the location of the needle as it is inserted. A locator device such as that described in Dumoulin et al. U.S. Pat. No. 5,271,400 issued Dec. 21, 1993 and U.S. Pat. No. 5,307,808, issued May 3, 1994, both of which are assigned to the instant assignee, may be used to track the location of the instrument and provide coordinate values to the MRI system which enable it to mark the location of the instrument in each reconstructed image. The medical instrument is attached to a handpiece that is manipulated by the physician and whose position is detected by surrounding sensors. For example, the handpiece may emit light from two or more light emitting diodes which is sensed by three stationary cameras.
Tracking devices which employ the MRI system to locate markers in the medical device have also been developed. As described in Dumoulin et al. U.S. Pat. Nos. 5,271,400, 5,307,808 and 5,318,025, Souza et al. U.S. Pat. No. 5,353,795 and Watkins et al. U.S. Pat. No. 5,715,822, each of which is assigned to the instant assignee, such tracking systems employ a small coil attached to a catheter or other medical device to be tracked. A NMR pulse sequence is performed to establish desired magnetic field gradients to produce transverse magnetization at the location of the tracking coil carried by the tracked device. The location of the tracking coil is determined and is superimposed at the corresponding location in a medical image acquired with the same MRI system.
During an interactive MRI diagnostic procedure or interventional procedure, it is common for the operator to frequently change the scan plane coordinates and orientation. It may also be desirable for the operator to modify the imaging pulse sequence parameters or exercise other control over the MR scanner in a rapid fashion, especially for interactive control of real-time imaging sequences. Additionally, it may be desirable for the operator to view in-bore the results of image formation on a screen built into the controlling device. There is currently no method that permits an operator working in-bore to perform such operations.